Setting a custom field as a required one is only available in the Premium plan.
When creating custom fields, certain types of custom fields can be set as required. Those include:
To set a custom field as required, head over to Settings - Custom fields:
Select the custom field you wish to update or create a new one:
Tick the option "Set as a required field":
Now if you try and create a new deal without the required custom field entered, an error will appear: